1. Field of the Invention
This invention relates to an array of ultrasonic transducers for use in a medical imaging apparatus. More specifically, the invention relates to a curvilinear, i.e., convex or concave, array of ultrasonic transducer elements which performs sector scanning of ultrasonic beams.
2. Description of the Prior Art
An array of ultrasonic transducers is used in a ultrasonic apparatus to observe the internal organs of a patient. Such an apparatus provides successive images at a rapid rate, in "real time", such that an observer can see movements of continuous motion.
A curvilinear array of ultrasonic transducers is disclosed in, for example, U.S. Pat. Nos. 4,344,327; 4,409,982; and 4,281,550. The former two patents disclose convex arrays and the letter patent discloses a concave array.
An advantage of these curvilinear arrays is the ability to perform sector scanning without a need for electronic sector scanning techniques to steer the ultrasonic beams over a large angle. In electronic sector scanning, plural ultrasonic transducer elements are linearly arrayed on a common plane. All the elements are excited at a different timing relation to phase the wave fronts of the respective ultrasonic waves to define a steered beam direction. But such excitation is liable to generate a side-lobe beam in addition to the main beam. The side-lobe beam gives the image an artifact because information obtained by the side-lobe beam is also interpreted as that of the main beam.
The curvilinear array of transducer elements performs the sector scanning of ultrasonic beams without exciting the transducer elements with different timing relations. Thus, an ultrasonic imaging apparatus using the curvilinear array does not need delay time circuits to give elements different timing relations to steer beams. Further, it provides a wider viewed image at more distant regions than obtained with conventional electric linear scanning.
It is, however, more difficult to assemble the curvilinear array relative to that of the non-curved, linear array because the piezoelectric ceramic plate for the ultrasonic transducer is rigid and is not itself flexible.
Therefore, a curved piezoelectric ceramic plate is fabricated by grinding a block of piezoelectric ceramic in a desired curvilinear shape. The thickness of the plate forming the array is about 0.3 mm to transmit 5 MHz ultrasonic beams. So it is not easy to grind the block to produce such a thin curved piezoelectric ceramic plate, especially of small radius. It is also difficult to divide the curved ceramic plate into the plural elements as compared with a non-curved one.
U.S. Pat. No. 4,281,550 discloses a concave array, wherein copper electrodes are bonded to the front and rear major surfaces of the plate with a silver bearing epoxy resin. A flexible matching window (layer) is then cast directly on the front electrode. A series of paralleled grooves are then cut through the rear electrode. The grooved ceramic plate is formed around a semi-cylindrical mandrel by cracking via each groove to produce a curved array of separate, electroded transducer elements.
But the fabrication shown in U.S. Pat. No. 4,281,550 is limited to a concave array because the grooved array can not be bent towards the grooved surface.